REST API: Preserve unknown, respect null in server-side block rendering.

- Skips validation where there is no attribute definition, but keeps the attribute value. Previously, the attribute would be omitted from the attributes passed to `render_callback`. Notably, this resolves an issue where `render_callback` cannot receive a block's `align` and `customClassName` attribute values, since these are defined as a client-side filter.
- Validates `null` as a proper value in its own right. Previously, a client implementation of a block could track `{“attribute":null}` as an explicitly empty value, and the server would wrongly initiate defaulting behavior. The new behavior will now only populate a default value if the attribute is not defined at all, including when unset in its being invalid per the attribute schema. 

Props aduth, noisysocks, youknowriad, danielbachhuber.

Merges [43918] to trunk.

See #45145 for the patch, #45098 for the original ticket.

git-svn-id: https://develop.svn.wordpress.org/trunk@44269 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php

@@ -61,6 +61,7 @@ class WP_REST_Block_Renderer_Controller extends WP_REST_Controller {
 								'type'                 => 'object',
 								'additionalProperties' => false,
 								'properties'           => $block_type->get_attributes(),
+								'default'              => array(),
 							),
 							'post_id'    => array(
 								'description' => __( 'ID of the post context.' ),